1. Field of the Invention
The present invention relates to a surface mount type quartz crystal oscillator, and a surface mount type crystal device.
2. Description of the Related Art
Surface mount type crystal oscillators, which have a quartz crystal element and an oscillation circuit using this crystal element, both contained in a surface mount package, are widely used as reference sources for frequency and time in compact portable electronic devices including, among others, portable telephones, because of their small size and light weight. An example of such surface mount type crystal oscillators is disclosed in US 2005/0193548A1.
FIG. 1 is a cross-sectional view illustrating an exemplary configuration a conventional surface mount type crystal oscillator. The surface mount type crystal oscillator has IC (integrated circuit) chip 2 and crystal blank 3 hermetically sealed in package 1. Package 1 comprises ceramic substrate 4 as a mount substrate on which components are mounted, and metal cover 5. Ceramic substrate 4 comprises a laminate of first ceramic layer 4a having a substantially rectangular and flat shape, and second ceramic layer 4b which has a substantially rectangular opening. The opening formed through second ceramic layer 4b defines a recess in one main surface of ceramic substrate 4. On a surface on a laminated side of first ceramic layer 4a, which corresponds to the opening of second ceramic layer 4b, stated another way, on the bottom surface of the recess in ceramic substrate 4, a plurality of circuit terminals 6 are disposed for electric connection with IC chip 2. On the back surface of first ceramic layer 4a, i.e., the outer bottom surface of package 1, mounting electrodes 7 are formed at four corners for use in mounting the crystal oscillator on a wiring board. Each mounting electrode 7 is also provided with an end face electrode 7b, i.e., an area formed to extend to a side surface from the outer bottom surface of first ceramic layer 4a. End face electrode 7b is formed by through-hole processing when ceramic substrate 4 is formed by laminating and burning ceramic sheets. Each mounting electrode 7 is electrically connected to circuit terminal 6 through end face electrode 7b associated therewith, and conductive paths, not shown, formed on laminated surface of first and second ceramic layers 4a, 4b. A soldering fillets are formed on end face electrodes 7b when the crystal oscillator is mounted on the wiring board by reflow soldering.
Sealing metal film 9 is disposed on the top surface of second ceramic layer 4b along the outer periphery thereof. A pair of crystal holding terminals 10 are formed on the top surface of second ceramic layer 4b along one side of the opening at positions corresponding to both ends of the side for holding crystal blanks. Crystal holding terminals 10 are electrically connected to circuit terminals 6 through conductive paths, not shown, formed on ceramic substrate 4.
As illustrated in FIG. 2, crystal blank 3 is, for example, a substantially rectangular AT-cut quartz crystal blank which is provided with excitation electrodes 13 on both main surfaces thereof, and lead-out electrodes 14 are extended from a pair of excitation electrodes 13 toward opposite ends on one side of crystal blank 3. Both ends of the side of crystal blank 3, to which lead-out electrodes 14 are extended, are secured to crystal holding terminals 10 by conductive adhesive 15 or the like, thereby electrically and mechanically connecting crystal blank 3 to ceramic substrate 4.
IC chip 2, which is substantially rectangular in shape, comprises at least an oscillation circuit, which uses crystal blank 3, integrated on a semiconductor substrate. The oscillation circuit is formed on one main surface of the semiconductor substrate through a general semiconductor device fabrication process. Therefore, a circuit forming surface refers to one of both main surfaces of IC chip 3, on which the oscillation circuit is formed, on the surface of the semiconductor substrate. The circuit forming surface is also formed with a plurality of IC terminals 8 corresponding to the aforementioned circuit terminals 6, as illustrated in FIG. 3. These IC terminals 8 include a power supply terminal, a ground terminal, an oscillation output terminal, a pair of connection terminals for connecting to crystal blank 3, an AFC terminal for receiving an automatic frequency control (AFC) signal, and the like. Then, IC terminals 8 are bonded to circuit terminals 6 through ultrasonic thermo-compression bonding using bumps 12, thereby securing IC chip 2 within the recess of ceramic substrate 4. This also causes the connection terminals of IC chip 2 to electrically connect to crystal holding terminals 10, and the power supply, output, ground, and AFC terminals electrically connected to mounting electrodes 7 associated therewith.
Metal cover 5 is formed in a concave shape, such that its opening end face is bonded to metal film 9 on second ceramic layer 4b, for example, through thermo-compression bonding using brazing metal 11 made, for example, of AuSn (gold-tin) eutectic alloy or the like, thereby bonding metal cover 5 to ceramic substrate 4. In this way, metal cover 5 hermetically seals IC chip 2 placed in the recess of ceramic substrate 4, and crystal blank 3 secured to ceramic substrate 4 within package 1.
In such a surface mount type crystal oscillator, since concave metal cover 5 is bonded to the outer periphery of ceramic substrate 4, the internal volume of package 1 can be made larger, and conversely, the crystal oscillator can be reduced in size while the internal volume of package 1 is maintained constant.
FIG. 4 illustrates a crystal oscillator which employs a flat metal cover. This crystal oscillator employs package body 1 made of laminated ceramics and having a step in a recess, and flat metal cover 5a, where metal cover 5a is bonded to package body 1a to hermetically seal IC chip 2 and crystal blank 3 within the recess. IC chip 2 is secured to the bottom surface of the recess, while crystal blank 3 has its one side secured to the top surface of the step in the recess and is thereby held in the recess. In such a crystal oscillator, from a viewpoint of manufacturing and the like, frame width d2 of the topmost layer of the laminated ceramic layers, in package body 1 must be equal to or larger than the height of the layer, thus causing this frame width d2 to be necessarily larger. Frame width d2 is, for example, 0.35 mm. Here, the frame width refers to the distance from an inner wall surface opposing the recess or opening to an outer wall surface in the ceramic layer having a recess or opening. Since crystal blank 3 is surrounded by the topmost layer of the laminated ceramic layers, package body 1a results in having outer dimensions larger than the size of crystal blank 3 further by a factor of two or more of frame width d2 in both of vertical and horizontal directions in the figure. On the other hand, when a concave metal cover as illustrated in FIG. 1 is used, a width needed to bond metal cover 5 to ceramic substrate 4 must only be ensured around crystal blank 3. Typically, this width is equal to thickness d1 of a metal plate which constitutes metal cover 5. This thickness is, for example, 0.08 mm. As such, the configuration illustrated in FIG. 1 can largely increase the internal volume of the package in the same outer dimensions and hence largely reduce the crystal oscillator in outer dimensions, as compared with the configuration illustrated in FIG. 4.
For manufacturing the surface mount type crystal oscillator configured as illustrated in FIG. 1, unburned ceramic sheets (green ceramic sheets) generally having a size corresponding to a plurality of the crystal oscillators are used. The ceramic sheets are then laminated and burned, and then are cut, after burning, into a plurality of ceramic substrates 4 each corresponding to one crystal oscillator. Specifically, as illustrated in FIG. 5A, second ceramic sheet 4B having an opening for each crystal oscillator is laminated on flat first ceramic sheet 4A which has been previously formed with an electrode pattern for each crystal oscillator, and two ceramic sheets 4A, 4B are laminated and burned together, and then are divided along division line X-X to produce individual ceramic substrates 4.
In this event, as illustrated in FIG. 5B in an enlarged view, wedge-shaped division groove 16 is cut into each of both main surfaces of the laminate of the ceramic sheets at the position of the division line, followed by the burning. Such division grooves 16 thus formed facilitate the division of the laminate of the ceramic sheets into ceramic substrates 4 for respective crystal oscillators after the burning. First ceramic sheet 4A corresponds to first ceramic layer 4a, while second ceramic sheet 4B corresponds to second ceramic layer 4b. 
However, when the ceramic sheets are baked after they have been formed with division grooves 16 as described above, a contractive force associated with evaporation of a binder from the ceramic sheets concentrates particularly on upper ends of a frame portion defined by second ceramic layer 4b, resulting in external force P exerted in a direction to reduce the area of the opening formed through second ceramic layer 4b. Such external force P thus produced causes first ceramic layer 4a to curve into a concave form in burned ceramic substrate 4. Resulting ceramic substrate 4 suffers from an exacerbated flatness, i.e., plane accuracy on the bottom of the recess.
Assuming here that IC chip 2 is secured to the bottom of the recess in ceramic substrate 4, which is a mounting substrate, i.e., the surface of first ceramic layer 4a, through ultrasonic thermo-compression bonding using bumps 12, sufficient pressure is not applied to bumps 12 on the concave surface which lacks the flatness, as illustrated in FIG. 5C, resulting in a lower strength exerted by blimps 12 for securing IC chip 2 to ceramic substrate 4. Particularly, in this event, those bumps 12 which were not applied with sufficient pressure can compromise electric contacts, and be more susceptible to peeling when an impact is applied thereto. In this connection, the flatness of the bottom surface of the recess in ceramic substrate 4 is preferably in a range of 10 to 15 μm or less.
Further, the surface mount type crystal oscillator configured as illustrated in FIG. 1 also has a problem of difficulties in reducing the manufacturing cost because it is fabricated by laminating second ceramic layer 4b having an opening on flat first ceramic layer 4a. Particularly, since a stamping process is required to form the opening through second ceramic layer 4b, the surface mount type crystal oscillator has a problem of a high processing cost. Another problem lies in that a limited area is merely provided for accommodating IC chip 2 because the surface mount type crystal oscillator receives IC chip 2 within the recess of ceramic substrate 4, and both ends on one side of crystal blank 3 are secured to the position of an edge of the recess. When a temperature compensation mechanism is additionally integrated on the IC chip in addition to the oscillation circuit in order to increase added values of the crystal oscillator, the IC chip is also increased in size. Accordingly, a certain area required on the top surface of ceramic substrate 4 for securing the crystal blank thereto constitutes a factor of impeding a reduction in size of ceramic substrate 4.
Surface mount type crystal devices which have a concave metal cover bonded to a ceramic substrate are not limited to the crystal oscillator described above. Such a concave metal cover bonded to a ceramic substrate can be employed as well in a crystal unit which has a crystal blank encapsulated in a package. Such a surface mount crystal unit is also utilized as a reference source for frequency and time in portable electronic devices.
FIG. 6 illustrates an exemplary configuration of a conventional surface mount type crystal unit. In the illustrated crystal unit, crystal blank 3 similar to that illustrated in FIG. 2 is mounted on flat ceramic substrate 4 which is substantially rectangular in shape, and concave metal cover 5 is bonded to ceramic substrate 4 to hermetically seal crystal blank 3 within a space surrounded by ceramic substrate 4 and metal cover 5. Ceramic substrate 4 may be, for example, in a two-layer structure, where metal film 9 is formed on the top surface of a second layer along the outer periphery thereof. A pair of crystal holding terminals 10 are also disposed on the top surface of the second layer. Mounting electrodes 7c are formed on the bottom surface of ceramic substrate 4, i.e., the lower surface of a first layer for use in mounting the crystal unit on a wiring board. Crystal holding terminals 10 are electrically connected to mounting electrodes 7c through conductive paths formed on laminated surface of the first and second layers, and a conductive film formed on the end face of the first layer. Then, both ends on the one side of crystal blank 3, to which lead-out electrodes 14 extend, are secured to crystal holding terminals 10 with conductive adhesive 16 in a manner similar to the aforementioned.
Metal cover 5 is processed such that an opening end face has a flange, and is bonded to ceramic substrate 4 by thermo-compression bonding through the intervention of eutectic alloy 11 such as AuSn alloy or the like between the flange plane and metal film 9. The flange of metal cover 5 is used to increase the width of a portion of metal cover 5 which is bonded to ceramic substrate 4, i.e., a so-called seal-path to ensure the bonding strength and air-tight sealing.
The crystal unit illustrated in FIG. 6 can be manufactured at a reduced cost by virtue of the employment of flat ceramic substrate 4, as compared with a crystal unit (see FIG. 7) which comprises crystal blank 3 placed in a recess of package body 1a, and flat metal cover 5a for closing the recess. In this connection, the crystal unit illustrated in FIG. 7 comprises metal ring 11a for seam welding disposed on the top surface of package body 1a to surround a recess, such that metal cover 5a is bonded to metal ring 11a by seam welding.
In the crystal unit illustrated in FIG. 6, eutectic alloy 10 is melted by heating, with an opening end face of metal cover 5 remaining in contact with eutectic alloy 10, to bond metal cover 5 to ceramic substrate 4. In this event, metal cover 5 is susceptible to shift in position to ceramic substrate 4, where part of the flange of metal cover 5, for example, can protrude from the outer periphery of ceramic substrate 4 to reduce the seal-path, possibly resulting in exacerbated bonding strength and air-tight seating. Also, with portion of the flange thus protruding from ceramic substrate 4, the resulting crystal unit can experience such problems as a failure in satisfying dimensional criteria related to the external shape, and a defective appearance.
Various problems caused by the shift in position between metal cover 5 and ceramic substrate 4 can also be experienced in the surface mount type crystal oscillator as illustrated in FIG. 1.